PDU in Physical Layer

Today I read a question in HVA asking about if there is a PDU in Physical Layer. The questioner read a Vietnamese book stating that there is no PDU in Physical Layer but he read in Wikipedia that the PDU is the Bit in Physical Layer. So he is a little confused to find out the answer and I will try to answer this question in this post.

PDU (Protocol Data Unit) has following meanings (as Wikipedia):

  • Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data.
  • In layered systems, a unit of data that is specified in a protocol of a given layer and that consists of protocol-control information of the given layer and possibly user data of that layer.

Network protocols are built usually in layers, for example the OSI layer model. They have different hierarchical layers processing different tasks. Each of them implement a protocol according to their task and is numbered with a number N.
Typically, each layer adds to the transmitted data its own management information. The complete set of data and information for management will be called Protocol Data Unit (PDU)  of this layer. The PDU is the full message of what the protocol layer N implemented.
If this PDU is now transmitted to lower protocol layer N-1, the layer N should send only the data what the layer N-1 is waiting for . The layer N-1 will provide the service to receive these PDUs. Therefore, the transmitted data (the PDUs of layer N) on the layer N-1 is called also the Service Data Unit (SDU) of the layer N-1.

The idea is shown very clearly in figure below.  If you take Physical Layer the number N, the Data Link Layer will take the number N – 1.

And a small formula for this concept can be
PDU(N) = SDU(N – 1)

= PCI(N) + SDU(N) + Footer(N)

= PCI(N) + PDU(N + 1) + Footer(N)

Where PCI is the Protocol Control Information.

So according to the figure below, the PDU of Physical Layer will be the Bits. The Protocol Control Information (PCI) can be the Preamble Sequenz, Start of frame delimiter…